The invention described herein relates generally to composite windows and more particularly to hydrofluoric acid-resistant composite windows and methods for their fabrication.
There are many industrial processes in which hydrofluoric acid is used. Examples are aluminum production, chlorofluorocarbon production, uranium production and petroleum alkylation for production of gasoline octane improvers. Windows are used to visually monitor these and other processes. A major problem in these processes is that the hydrofluoric acid etches the exposed surfaces of the windows resulting in reduced visibility. It can be expensive to replace the windows frequently and, when gloveboxes are used, the impaired visibility reduces operator efficiency.
In facilities that produce enriched uranium and plutonium metals using fluorination followed by reduction, the reduced visibility poses a dual problem. Not only is operatior efficiency reduced because of impaired visibility, but personnel radiation exposure is increased because of the necessary increase in time spent working in the gloveboxes. Consequently, it is highly desirable to design a protected window with a long service life for use in processes involving hydrofluoric acid.
U.S. Pat. No. 4,433,893 to Yokota discloses a process for producing glass holograms having an uneven surface. The diffraction efficiency of the hologram is increased by exposing the hologram to dilute hydrofluoric acid. When producing a transmission-type glass hologram, the back surface of the glass hologram can be protected from the hydrofluoric acid by covering the back of the hologram with an adhesive tape having a Mylar base. The Yokota process is distinguishable from the present invention in several ways. The Yokota process increases the diffraction efficiency of a hologram by applying hydrofluoric acid to the hologram and etching its surface. The present invention protects windows from hydrofluoric acid by covering the exposed side of the windows with a layer of hydrofluoric acid-resistant transparent material. Yokota discloses the protection of the back of the glass hologram with an adhesive tape comprising Mylar. There is no mention of the need for transparency in Yokata. There is no need to visually inspect a process in Yokota. Yokota is not concerned with the protection of a glass window with transparent material to permit a process to be visually monitored. The process disclosed by Yokota involves hydrofluoric acid treatments lasting no more than two and one-half minutes. Yokota is not concerned with long-term protection from hydrofluoric acid. Moreover, Yokota is not concerned with abrasion resistance or protection from concentrated hydrofluoric acid. Yokota merely teaches that an adhesive tape made with Mylar can protect one side of a hologram from dilute hydrofluoric acid for short periods of time.